Recently image capturing, displaying and in particular encoding has improved from so-called low dynamic range (LDR) imaging (such as used for well known classical, standardized systems like PAL or MPEG2) to so-called high dynamic range imaging (HDR). Illuminances in nature can range from 100,000 lux in sunlight, over typical office or room illuminations of around 500 lux, to e.g. 0.05 lux under quarter moonlight. Luminances (L) in the world range from 1 billion nit of the sun disk, to 10,000s of nits for lamps, to a couple of (ten) thousands of nit for objects in sunlight (like a building, cloud rims, or a white paper), to hundredths or 10s of nits for objects under (heavily) overcast sky or indoors, to 0.1 nit for a white paper under moonlight, etc. This doesn't necessarily mean one should render these luminances on a display in exactly the same way, rather, the picture should look artistically good, meaning at least that there should be approximately similar appearance differences for the regional luminances of objects when rendered on the display screen.
A further extension of image quality may be achieved by extending the color space compared to the classical standardized video, usually called LDR systems. The extended range video as discussed in this document has an extended brightness range, an extended color space, or a combination of an extended brightness range and extended color space, and may be called HDR video.
One should understand that tone mapping for rendering on a particular display is with the many displays existing nowadays decoupled from capturing or coding, leading to three linked representations. In general, a requirement of HDR imaging to be able to render e.g. a bright white wall differently from an adjacent bright lamp on a display, is that their respective pixels are also encoded with different luma (Y) values. Sensors or cameras are becoming more powerful in that indeed they can capture most of those many different luminances and/or colors in the world faithfully (whether with larger well-depths, differently exposed pictures, etc.), and for simplicity we shall consider their native color representation to be a linear luminance encoding within [Lmin, Lmax]+chromatic information. We may then use an entirely arbitrarily specified definition (according to desired requirements of course, such as e.g. later processability of the encoded information like local brightening, or data compression concerns, etc.) for our transmission codec. Lastly this encoded data (Y_C1C2, or similar) can then again be converted in many ways to a rendering-side representation, which we can for simplicity equate with driving values for e.g. the LCD pixel colors. New displays may have more renderable dynamic range, so that they can firstly render more bright regions, and secondly simultaneously or successively more dark regions. Such extended range allows placing all these various luminance objects along the renderable gamut with optimal rendered output colors. Also the available color space may be extended.
Blu-ray discs and players were introduced in the market in 2006 and since then millions of players, including PlayStation 3 game consoles, have been sold. The Blu-ray discs (BD) mostly contain a 2D or 3D feature movie with a playing time of around 2 hours and additional content (bonus material). The video content on these discs has been color graded for relatively low brightness levels and a color matrix defined in ITU-R recommendation BT.709 “Parameter values for the HDTV standards for production and international programme exchange”, available from: http://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.709-5-200204-I!!PDF-E.pdf.
Recently, the broadcast and content industry is showing a lot of interest to move from Full HD resolution to double horizontal and vertical resolution (3840×2160). This quad Full HD (QFHD) resolution is sometimes referred to as 4K. In addition to increasing the resolution, there is also a wish to move to higher frame rates (48 Hz, 60 Hz), higher bit depth for coding the color components, higher dynamic range and wider color gamut. Together the higher quality video specifications are often referred to as Ultra HD. See Rec ITU-R recommendation BT.2020, available from: http://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.2020-0-201208-I!!PDF-E.pdf. Meanwhile a new codec was developed in a joint effort by ITU-T and MPEG, dubbed HEVC (a.k.a. ITU-T H.265 or MPEG-H Part 2), which is generally considered to be about twice as efficient as AVC.
Display devices with higher brightness levels and/or wider color gamut as discussed above are appearing in the market. The non-standardized brightness levels can be anything from (say) 200 nits to 1000 nits or higher. Existing Blu-ray content is graded for lower brightness and to utilize the higher brightness the HDR displays may apply some algorithm to stretch the brightness of the content. The result may not be ideal from a creative point of view. One way to take into account the creative intent when stretching to higher brightness is to include information with the content to tell the player (or the display) how to do the stretching. This stretching may vary during playback (e.g. change per scene), or may even be spatially adapted. Also, the stretching may be different for the video and the overlaying graphics (e.g. subtitles and menus). Examples of how this information may look like are described in patent application WO2013/046095, which describes a system for dynamic range transformation of images. A dynamic range transform, i.e. a conversion processing schedule, is discussed for converting encoded video between LDR video and HDR video for a target HDR display. A target display reference may be included in the output image signal. Also a further document WO2012/147022 describes mapping between HDR and LDR video, which mapping may be defined by predefined mapping algorithm data. Further examples are provided on how to use this information to optimize the processing of the video or the graphics or both.
From the above it follows that generating HDR video and converting between HDR and LDR video according to video processing metadata representing a conversion between the HDR video and the LDR video, are known as such. However, a considerable problem for content creators is that many users still have legacy equipment that cannot handle the extended luminance and/or color range provided by the HDR material.
WO2012/021705 discloses systems and methods for content delivery of at least two versions. The transfer signal has a first version of the content, a difference data representing a difference between the first version and second version of content, and metadata derived from transformation functions that relate the first and second version to a master version.
EP 1708485 discloses a transfer format for video data between a display device (TV receiver) and a processing device (optical disc player). The display device provides a reception specification (EDID signal) that defines the signals that can be displayed. The processing device generates the corresponding version (SD or HD) of the content.